Op-amp

Op-amps such as the 741 and similar designs are rapidly replacing individual transistors in circuit diagrams that the reader is likely to see and wish to use. Newer types are available with FET transistors, and some contain both FET high-resistance inputs and high-current bipolar outputs. Generally, op-amps are the building blocks of many industrial circuits, and they should become another part of the reader s intellectual toolbox.  

An easier-to-use op-amp is the LM386. (This is no relation to the Intel 386 micro-processor IC, which is not an op-amp, is digital rather than analog, and has CMOS transistors.) The 386 needs only one power supply and can put out more current — enough to drive a small loudspeaker without a buffer amplifier. The experimenter should build the circuit of Fig. 23.3, with an infrared-sensitive photodiode and a small loudspeaker. It will enable the pulse code modulated (see index if necessary) lightwave signals transmitted by a remote control for a TV or DVD set to be made into audible sound signals. (We could use a 4.7 mfd capacitor between pin number 5 and the loudspeaker, to prevent dc output, but that is not necessary in this simplified circuit.) 

The 386 is in the form of a dual in-line package (DIP). The black material is injection-molded epoxy polymer, with carbon black added to prevent random light from putting stray voltages on the PN junctions of the IC. Two lines of leadwires stick out of the bottom, like the legs on a centipede insect. It is difficult to attach clip leads to these, so a breadboard socket module will be used. This will provide useful experience for the experimenter, since these modules can be helpful in future circuit building. 

Looking at the top of the DIP, with the leadwires facing away from the experimenter, the circuit diagram of what is inside the package is shown in the next diagram, Fig. 23.4. The pin numbers identifying the leadwires should be compared to those numbers in Fig. 23.3. 

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The schematic and Bode plot for the single-pole method of compensation are given in Figure B-16. At dc it exhibits the full open-loop gain of the op amp, and its gain drops at -20dB/decade from dc. It also has a constant -270 degree phase shift. Any phase shift contributed by the control-to-output characteristic... 

The minus sign indicates that the phase is reversed. For / fb = 100 MO, one nanoampere of input current results in an output voltage of 100 mV. In some cases, the sample must be grounded, then the noninverting input of the op-amp may be connected to a fixed dc voltage as the bias. The output voltage is the... 

Fig. 11.1. Two basic types of current ampliflers. (a) Feedback picoammeter. It consists of two components, an operational amplifier (op-amp) A, and a feedback resistor 1 fb- a typical value of the feedback resistor used in STM is 10 fl. The stray capacitance Cfb is an inevitable parasitic element in the circuit. In a careful design, Cfb 0.5 pF. The input capacitance Cm is also an inevitable parasitic element in the circuit. Those parasitic capacitors, the thermal noise of the feedback resistor, and the characteristics of the op-amp are the limiting factors to the performance of the picoammeter. (b) An electrometer used as a current amplifier (the shunt current amplifier). The voltage at the input resistance is amplified by the circuit, which consists of an op-amp and a pair of resistors R, and R2. The parasitic input capacitance Cm limits the frequency response, and the Johnson noise on Rm is the major source of noise. Also, the input resistance for this arrangement is large.

Another limiting factor is the bandwidth of the op-amp. On the factory specifications, the commonly used indicator is the gain-bandwidth product. The nominal dc gain is valid up to a cutoff frequency / which is typically 10 Hz. Above that frequency, the gain g is inversely proportional to the frequency. The product of gain and frequency, the gain-bandwidth product/a is typically 1 MHz. The input impedance of the amplifier increases with frequency ... 

Fig. 11.3. The influence of the input capacitance on output noise. To make a simple estimation, the input noise of the op-amp is represented by an ac source at the noninverting input end. The smaller the input impedance, the larger the noise at the output end. Therefore, the input capacitance generates a large high-frequency noise.

Fig. 11.6. Simple feedback electronics with integration compensation. The first op-amp amplifies the error signal with a variable gain. An RC network provides an integration compensation. A high-voltage op-amp provides an output of 100 V or more, to drive the z piezo.

Therefore, leakage due to the amplifier after the capacitance is not a problem. The problem is the switehing device before the capacitor. Feenstra et al. (1987a) used a reed relay as the switching device. The open-circuit resistance of a reed relay is larger than 10 il. The leakage current is even smaller than the input of the FET op-amp. (Experience has shown that the printed circuit... 

Fig. 14.1. Electronics for local tunneling spectroscopy. By using an op-amp with FET input stage as the isolation amplifier to the high-voltage amplifier for the z piezo, the holding time on the capacitor can be as long as 100 sec. The values of R and C show typical ranges.

The operational amplifier or in short, op-amp, is used so extensively in modem electronic circuits that it is called a panacea. Op-amps are always used with negative feedback so that the circuits are essentially determined by the feedback networks only. Within certain limits, the characteristics of the op-amps can often be neglected (Fig. H.2). 

Figure H.l shows two simple applications of the op-amp. In the inverting amplifier, the output voltage tends to make the voltage difference between the inputs zero while keeping the input current zero Therefore,...

Fig. H.2. The operational ampliiier (op-amp). It has two inputs with very high impedance, an output with very low impedance, and very high gain.

Fig. H.I. Two simple applications of the op-amp. (a) An inverting amplifier, (b) A noninverting amplifier.

Tables H-1 and H-2 provide information on some useful op-amps in STM and AFM. Table H-1 lists op-amps for tunneling current amplifiers. The requirements are, low bias current h, low input noise level, i and e . The typical power supply voltage is 5 - 18 V.

Table H.l. Op-amps recommended for tunneling current amplifiers ...

Table H-2 lists high-voltage op-amps for driving the piezos. The requirements are a high supply voltage range and a high slew rate (SR). The usable current Iq is also an important parameter for high-voltage op-amps. In STM, it is not critical. Most of the useful high-voltage op-amps are manufactured by Apex.

The op-amp constant current source below is designed to eliminate the effects of temperature on the BJT used in the current source. This current source is a very accurate and temperature-independent current source ... 

This example uses the non-ideal op-amp model of an LF411C. Since there is only one op-amp in this circuit, we will not reach the component limit of the Lite version. The method described here can be used for circuits with several op-amps. However, the component limit of the Lite version of PSpice limits us to only two non-ideal op-amp models. If more than two op-amps are needed, the Ideal OPAMP model can be used. Since the Ideal OPAMP model has no frequency limitations, it cannot be used to find the bandwidth, but it can be used to find the gain. 

In this section we will use PSpice to determine the bandwidth of an op-amp circuit with varying amounts of negative feedback. For an op-amp circuit, the closed-loop gain times the bandwidth is approximately constant. To observe this property, we will run a simulation that creates a Bode plot for several different closed-loop gains. We will use the circuit below ... 

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